1. Field of the Invention
The present invention relates generally to a capacitor, and more particularly, to a capacitor with a sealed sheet of separator enclosing the capacitor anode.
2. Prior Art
The current trend in medicine is to make cardiac defibrillators, and like implantable devices, as small and lightweight as possible without compromising their power. This, in turn, means that capacitors contained in these devices must be made with minimal size while still meeting the power and energy requirements of the devices. In general, an optimum capacitor design provides as much cathode and anode active materials within the capacitor casing as possible. In order to accomplish this, the volume needed for non-capacitive components, such as separators, insulating rings, connecting and feedthrough wires, and a glass-to-metal seal should be minimized.
In an electrochemical capacitor, a separator is disposed between each opposed anode and cathode surface to prevent an internal electrical short circuit between the opposite polarity active materials. FIGS. 2 to 4 illustrate a prior art capacitor 10 comprising a separator 12 enveloping an anode pellet 14 of a commonly used shape. The separator 12 is preferably formed as a pouch that encloses the anode 14 isolated from the cathode active materials 16 and 18 supported on the interior surfaces of the casing members 20 and 22, respectively. In fabrication, sheets 24 and 26 of separator material are placed in contact with the anode 14 and heat sealed to each other by a hot press 30 (FIG. 3) near the lower perimeter 28 of the anode 14. Excess separator material (not shown) is cut away (if necessary) during or after the heat sealing process, leaving a selvage flap 32 around the anode pellet 14 that consists of a double layer of the fused separator sheets 24, 26.
Casing member 20 is made with the minimum volume required to receive the anode pellet 14 enclosed in the separator 12. The separator selvage flap 32 is folded upwardly when the anode 14 enveloped in the separator 12 is disposed in the casing member 20. The selvage flap 32 is contained in a gap 34 formed between a portion of the separator 12 that is contiguous with the sidewall 33 of anode 14 and the sidewall 36 of casing member 20. This gap 34 is present around the entire anode sidewall 33 and constitutes wasted space within the capacitor 10 that does not contain electrode active material.
Thus, current separator sealing methods result in the relatively thick separator selvage flap 32 that necessitates providing the gap 34 within capacitor 10, thereby lowering capacitor volumetric energy density. What is needed, therefore, is a method of sealing the separator material around a capacitor anode that either provides no selvage flap, or a selvage flap of minimal thickness, thereby reducing or eliminating the associated wasted space in the capacitor.